1. Field of the Invention
The present invention relates to a new method for making microlenses that can be used in optical applications.
More precisely, it relates to microlenses made on a flexible or rigid, organic, metallic or semiconductor substrate.
A lens is generally an optical device capable of modifying the convergence of a light beam by means of a curvature of one of its faces. It is formed by a portion of a sphere and is defined by its focal length, which is dependent on the refraction index of the material and on the radius of curvature of the sphere.
Associated in groups, microlenses can be used for example in displays to increase the inter-pixel space so as to facilitate the implanting of the control tracks for the pixels or they can be used in optical reading devices or X-ray imaging devices. In the latter two types of applications, the matrix of microlenses is used to make photon beams converge on photoconductive elements that are capable of transmitting information elements.
2. Description of the Prior Art
There are several techniques for the manufacture of microlenses at the present time:
The firm Corning uses a technique that enables the batch manufacturing of the lenses by subjecting a photosensitive glass to a UV radiation through a mask. The glass then undergoes a heat treatment at a temperature of 500.degree. C. to 600.degree. C., leading to a crystallization of the zones subjected to ultra-violet rays and, hence, locally to a densification of the material. This densification causes a contraction of the dimensions that forces the non-illuminated zones to emerge in spherical form. The drawback of this technology is the nature of the substrate which dictates the use of a special photosensitive glass that is rigid and cannot be used to cause variation notably in the focal length of the microlens thus formed.
Nippon Sheet Glass Co. has developed microlenses by using the technique of ion exchange which consists in exchanging an ion of the substrate with an ion of a melted salt previously deposited through a metal mask. The substrate is identical to the one used by Corning. It is a silica containing different oxides capable of migrating at temperatures of over 500.degree. C. At these temperatures, the ions of the melted salt diffuse from outside inwards while the ions of the substrate diffuse from inside outwards, and this leads to an index gradient. The drawbacks of this technique lie in the duration of the method, which lasts about 100 hours and is conducted under very high temperatures (500.degree. C.).
Xerox has developed another technique in which there is deposited, on a quartz substrate, an aluminum structure wherein only disks with a diameter d are left transparent. Centered on these transparent zones, there is deposited a positive resin pattern with a diameter D, such that D is greater than d. After the inversion of the resin by deep UV treatment, a cylinder pattern is obtained with a diameter D' between d and D. Several types of thermal and UV treatment (Applied Optics, vol. 27, No. 7, 1988) enable the cylinders to be converted into spheres. The advantage of this technique lies in the use of the standard methods of microelectronics, but the major drawbacks remain the number of steps and masks that have to be used for this method as well as the problem of achieving control over the dimensions of the patterns and over the resistance of the lower layers to the chemical solvents used.
In general, these techniques cannot be used to make microlenses on varied substrates such as semiconductors, polymers, metals or crystals, but only on rigid substrates that are limited in nature. Furthermore, most of them have complex protocols with numerous steps.